Evaluation of alternative ELISA methods and immunogenicity tests in small animals for determining potency of Poliovirus Vaccine Inactivated (IPV) based on D antigen determinations, has been conducted in collaboration with WHO, vaccine manufacturers, and other national control authorities. The variable results obtained in these collaborative studies (Wood DJ, Heath AB, and Sawyer LA, Biologicals 1995;23:83-94,Wood,D.J. and Heath, A.B. Biologicals 1995;23:301-311) indicated the need for further improvements in in vitro determinations of IPV potency. Work is continuing of the improvement of the CBER developed monoclonal ELISA and the evaluation of alternative ELISA procedures, with the goal of developing a suitable method for IPV potency determinations that is acceptable to CBER, manufacturers, and other regulatory agencies. Recent results have provided further evidence of the importance of evaluation of test results in the determination of the potency of inactivated poliovirus vaccines. This research is being pursued in order to develop an improved potency assay for IPV. The assay currently used to determine potency, as required by the manufacturer's license, measures the serological response of monkeys to inoculation with three doses of the vaccine. The high potency IPV vaccines available today easily pass this potency assay. However, the monkey test suffers from the limitation that the results are hard to quantitate beyond pass/fail. Every manufacturer of IPV also uses some form of ELISA procedure to follow the antigenic properties of the vaccine during manufacture. Development of a standardized ELISA procedure that would improve the quantitative analysis of the vaccine and could be used for accurate potency determinations and eliminate the use of monkeys and provide a better vaccine. Potency determinations of IPV will become of increased importance as the US switches to increased reliance on IPV in the national immunization program.